Restrict param type of AdminInterface identifiers methods

[VincentLanglet]

Subject

I am targeting this branch, because BC.

I'm introducing deprecation related to this PR: #6304

Changelog

### Deprecated
- Passing another type than `object` to `AbstractAdmin::toString`
- Passing `null` to `AbstractAdmin::getUrlSafeIdentifier`, `AbstractAdmin::getNormalizedIdentifier`, `AbstractAdmin::getId`
- Using `GetShortObjectDescriptionAction` with an empty objectId. 

[phansys]

The deprecation path seems right to me. We need to fix the QA checks.

[VincentLanglet]

The deprecation path seems right to me. We need to fix the QA checks.

I have two solutions:

• Removing the deprecations since the interface does not allow null
• Changing the interface type to allow null

Theorically null is not allowed but all our storages was allowing null, so I don't know what is the best choice.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?